This invention relates to a color corrector in a color image reproducing apparatus for displaying/printing a color image on the basis of color signals or color data (hereinafter referred to as video signals) corresponding to primary colors such as Red (R), Green (G) and Blue (B), etc., and more particularly to a color corrector in a color image reproducing apparatus for converting video signals of R, G and B to recording density signals or data (hereinafter referred to as recording density signals) of Cyan (C), Magenta (M) and Yellow (Y) which are complementary colors thereof, respectively.
For example, in color Televisions (TV), an approach is employed to represent a color image as sum of respective components of the three primary colors respectively having predetermined chromaticities to further convert electro-optic conversion characteristics of emitting bodies of respective primary colors on the reproducing side to electric signals to which correction (which is called a .gamma.-correction) is implemented. Accordingly, in reproducing such electric signals, three primary colors having the same chromaticities and conversion characteristics as those used at the conversion to electric signals are used in principle.
In practice, for the reason why it is difficult to increase the brightness of a display image, there are instances where primary colors of which chromaticities deviate from the above-mentioned predetermined chromaticities are used as a light source for regenerative display. Further, in the case of providing a hard copy from the above-described electric signals by using a printer, an approach is employed to convert signals corresponding to the above three primary colors to signals corresponding to Cyan, Magenta and Yellow (the subtractive primaries) which are respectively complementary colors thereof to convert .gamma.-corrected signals to original ones (inverse .gamma.-correction) so that density signals corresponding to respective complementary colors are provided to carry out printing by using these density signals thus to reproduce or reconstruct an image. Also in this case, it frequently takes place that inks used have the relationship that their colors are not completely complementary to the original three primary colors (R, G, B).
In the case where the chromaticities of primary colors used at the time of reproduction or reconstruction of an image deviate from desired chromaticities as in the above-mentioned example, colors of a display image or hard copy are reconstructed or reproduced in a manner that their chromaticities also deviate from those of the original image. In order to correct such a deviation, a color corrector is used.
A conventional example of a color corrector for printer used for the above purpose is shown in FIG. 1.
This color corrector circuit 50 comprises a frame memory 51 for storing color video signals R, G and B corresponding to respective primary colors (Red, Green, Blue), an inverse .gamma.-correction/luminance-density conversion circuit 52 for implementing an inverse .gamma.-correction to color video signals R, G and B read out from the frame memory 51 to provide signals of R.gamma., G.gamma. and B.gamma., and to convert these .gamma.-corrected color video signals R.gamma., G.gamma. and B.gamma. to optical density signals DR, DG and DB of Cyan, Magenta and Yellow which are respectively complementary colors thereof, and a color masking circuit 53 provided with the optical density signals DR, DG and DB to output printing or recording density signals C, M and Y.
As previously described above, since, in most cases, inks (dye stuffs) of respective colors of Cyan, Magenta and Yellow used in a printer have not desired optical characteristics as the primary colors, for example, a component of a primary color is mixed with other colors, when printing is simply carried out in accordance with the optical density signals DR, DG and DB, a hard copy having a chromaticity deviating from that of the original image may be provided.
The color masking circuit 53 carries out a signal processing based on the color masking matrix expressed as the following equation (1) in order to lessen the influence of the above-mentioned contamination color, thus to output recording density signals C, M and Y such that a reproduced color on a hard copy has a density close to the density corresponding to optical density signals DR, DG and DB. ##EQU1##
Generally, the color correction coefficient Aij shown in FIG. (1) is set by using various least square methods. These least square methods are described in detail in, e.g., "Journal of the Society of Image Electronics" Vol. 18, No. 1, 1989, pp. 20 to 28.
However, with a method of setting the correction coefficient Aij so that each reproducibility of intermediate colors becomes good by the least square method as shown in the above example, there are instances where the gray balance of a reproduced color on a hard copy or a display image is collapsed or destroyed.